Automatic gain controlled amplifier



Oct. 21, 19419 ROWN 2,259,998

' AUTOMATIC GAIN CONTROLLED AMPLIFIER Filed July 25, 1940 Inventor: George M. Bro n,

His Attorney.

Patented Oct. 21, 194i U I E-D STAT AUTOMATIC GAIN CONTROLLED AMPLIFIER George M. Brown, scotia N. Y.,;assi gnor to General Electric Company," a corporation of New" York Application July 23, 1940, Serial Ndlsiaais Claims. (o 179-471) j 'M y invention relates to amplifiers and has for an object to provide an improved high speed gain control means therefor.

' Many amplifiers employing automatic gain control means require that such means be capable of acting very quickly in order to prevent momentary increases in the output-of the amplifier upon very suddenincreases in the input to'the amplifier. It is particularly desirable, for example, inflamplifi'ers-"employed as modulators in radio transmitters to avoid any possibility of such momentary increases in output in modulating amplifiers, in order to prevent 'overmodulation in the accompanying transmitter.

'lin modulating amplifiers, automatic gain control potential is derived from the amplified sig: nal. If sufficient filtering be provided to prevent oscillation and distortion through the amplifier and'the automatic gain control circuit, the gain control action is slow unless unusualprecaution be taken, such as by providing a balanced or pushpull stage to be controlled by the automatic gain control potential, or by providing a special type'of filter. It is accordingly afurther object of my invention to provide high speed gain control means for an amplifier in which such'unusual precautions need not be resorted to and in which no unusual circuit elements'need be'provided.

."The features of 'my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however,

both as to its'organization and method of operation, togetherwith further objects and advantages' thereof mayTb-est be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure of the drawing represents ,an amplifier embodying my invention. 7

In the figure-an audio signal from amicrophone H] is amplified successively through electron discharge amplifier devices l2 and I3 to energizea load device- M, which is represented as a resistor but maybe any device to be energized by an amplified audio signal, such as. a radio transmitter, aloud speaker or the like.

.The dischargedevices II and I2 are represented as having respectively, control electrodes |5 and Iii-upon which audio signals from the microphone |-|l appear, and additional control electrodes H and J8, whose continuous potential maybe adjusted to-change the gain of the respective discharge device. Conventional means, not described, is employed to connect the discharge-devices ,|2 andv I3 in cascade to amplify audio; signals from. the microphone Hi.

used for automatic gain control purposes.

The discharge device I3 is of the so-called duplex-.diode-triode type in which a diode-is provided to rectify the amplified audio, signal to produce a unidirectional potential, which may be An anode IQ of the triode section of the device l3 transmits amplified audio signals tothe load device 4, and is also connected through a condenser 20; to an anode 2| of the diode section ofthedevice I3. .A common cathode 22 of. the device |3 is connected through two serially connected resistors 24 and .25 to ground and to. the negative terminal of a source 23. of potential. The positive terminal of the source 23 is connected through the load device M to the anode I9. The two resistors 24 and 25 are shunted for audio frequency currents by a by-passing condenser. 26. The anode 2| is connected through a voltage dividing resistor 21 to ground.

The cathode 22 is maintained at an average positive potentialwith respect to anode 2| by space current flowing through the anode I9, the cathode 22, and through resistors 24 and 25, to delay production of-auto-matic gain control potential across the resistor 21. The anode 2| of the gain control diode rectifier must therefore be made more positive with respect to ground by amplified audio voltage transmitted thereto from the anode |9through the condenser 2|] before the diode rectifier can become conductive and before space current can pass between the anode 2| and the cathode 22. No gain control potential is therefore produced across the resistor 21 until the anode 2| conducts space current at a time when the amplified audio signal rises above a certain predetermined level. This predetermined level may be controlled by resistor 25, which adjustable. As the audio signal on the anode l9 rises above this level, the gain control potential across the resistor 21 rapidly becomes more negative with respect to ground.

Thegain control potential produced across the resistor 21' is a pulsating voltage, and it is applied to the gain control electrodes I! and l8 of discharge devices H and I2 through appropriate filters. For example, the anode 2| is connected througha resistor 29 to the control electrode l8 of discharge device l2, which electrode is bypassed to ground through a condenser 30. The resistor 29 and the condenser 30 operate as a filter to smooth the ripple on the gain control potential across the resistor 27 to a certain extent. 1

The filter 29, 30 should allow the gain control potential to actvery quickly and when so adjusteol will smooth the ri ple on the gain control potential in small degree. The filter should slow the action of the gain control potential only enough to avoid making the volume of the audio signal uniform, so as to impair its intelligibility. In other words, the time constant of the filter 29, 30 should be made just sufficient to avoid distortion of the audio signal. Under such conditions of adjustment, the filter 29, 30 transmits the control potential with sufiicient speed to avoid momentary increases in the output of the gain control amplifier.

The gain control electrode I'Iof discharge device II is connected through a switch 3i and a resistor 32 to an adjustable tap of the voltage dividing resistor 21. The electrode I1 is bypassed to ground through a condenser 33. The resistor 32 and the condenser 33, acting as a filter, cooperate with the electrode I! in the same way as the filter 29, 30 cooperates with the electrode I8. The filter 32 33 should operate at. the same high speed, that is, with the same short time constant, as the filter 29, 3'6. An increase in. the gain control potential across the resistor 21 decreases the gain of both discharge devices II and I2.

Since the filters 29, 30 and 32, 33 are made to allow the gain control. potential to act with. as high speed,.or with as small a time delay, as possible While maintaining audio fidelity, there is a substantial amount of ripple voltage transmitted. from the anode I9 of the discharge device I3 through the condenser 20 to the electrodes I] and I8. This ripple voltage. afiects the space current in both of the devices II- and I2. Since these two discharge devices are connected successively in cascade, however, and the ripple voltage of the electrodes II'and I3 is in. phase, the ripple voltage on the electrode I'I affects: the space current in the device I2 oppositely to the efiect on that space current of the ripple voltage on the electrode I8 of the device I2.

The effect of the ripplevoltage on the space current in the device I2, and consequently on the output of the amplifier, is conveniently cancelled by adjustment of the tap on the voltage dividing resistor 27. At the proper adjustment of the tap on resistor 27, the ripple on the electrodes I! and N3 of the discharge devices II- and I2 is made to affect the space current in the device I2 in equal amounts as well as in opposite sense.

The two filters 29', 39 and 32, 33 should have the same frequency characteristic measured through the two paths from the resistor 21 to the device I2, respectively, through resistor 29, and

through resistor 32 and device, H. In other Words, a wave of any particular frequency'should be transmitted with a certaintime delay from the resistor 21 through the filter 2 9, 30 to the an-- ode of the device I2, and. with substantially equal time delay from the resistor 21 through the filter 32, 33, the device I I, the resistor 34, control electrode I5, and thus to the anode of thedevice I2. Provision for such equal transmission time in the two paths assures that any ripple voltage on the resistor 27 shall afiect the anode ofthe device I2 in exactly opposite phase through the two paths.

Adjustment of the amount of ripple voltage'fed from the resistor 2? through the device H to the device I2 may alternatively be adjusted by an adjustable tap of a resistor 34 upon which signals from the device I I are impressed. The control electrode I6 is connected to the adjustable tap of resistor 34, so that't-he-amount of signal tra-nsmitted from the device I I to the control electrode I6 may be controlled thereby. The amount of signal thus transmitted may be reduced sumciently, at a particular value of control potential on the resistor 21, so that the ripple voltage transmitted from resistor 21 through the device II to the device I2 is equal and of opposite phase to the ripple voltage transmitted through the resistor 29 to the device I2.

While the first of these methods of adjustment is preferred, either is subject to a disadvantage.

When the control potential on the retistor 21 changes, the gain. of the device II changes, so

be adjusted to apply all of the signal from device if to the control electrode I 6-, so that the amount of the ripple voltage transmitted to the device. II is adjusted by the tap: of resistor 21.

An alternative connection of the amplifier is.

provided which cancels ripple. voltage in the output completely even when thepotenti'al across the resistor 27 is changed. This alternative connection exists when the switch 3'I' is moved to itslefthand position, so that the control electrode I1 is connected through a resistor 35' to ground. A constant bias voltage exists between the electrode i7 and cathode 3'! or" device I I produced. by flow of space current from cathode '37. to ground through a resistor 38 by-passed for audio frequency currents by a'condenser 39. In this position of the switch 3| an adjustable amount of the ripple voltage across the resistor 21 is transmitted through the resist'or32 and through a large condenser 36, connected in shunt to the switch 3I, to the control electrode I-I;

Adjustment of. the amount of ripple voltage withthis connection may be made by'theadjustable tap either onthe resistor 27 or on the resistor 36. It is desirable, however, in order to have maximum gain for the audio signal from the microphone Ii), to move the' adjustable tap from the resistor up, so as to transmit'a maximum: amount of signal from the device II to the control electrode I6. Theadjustment of the-ripple voltage is then preferably made by adjustment of the tap of the resistor 21.

By the provision of my improved highspeed gain control means in an amplifier including two successive cascaded amplifying devices, so that ripple on the gain control potential affects the signal passing through the twosuccessivedevices tering necessary in the automatic gain control circuit to an amount just sufiicient toavoid impairment of the audio signal itself. While the use of two successive cascaded amplifiers is preferred,.it isiwithin the. scope of my invention toapply a gain control potential to any two cascaded amplifiers while maintaining the phase of the ripple voltage thereon such that it does not affeet the amplifier outpu While ,I have. shown. a particular embodiment of my inventiomit will, of course,.be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed, and I aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a pair of cascade connected alternating current amplifying devices operating in phase opposition, means for adjusting the amplification of each of said devices in response to a gain control potential, means to transmit to at least one of said adjusting means a control potential having an undesired pulsation thereon which produces an undesirable effect on the output of said devices, and means to transmit an amount of said pulsation through the other of said adjusting means sufficient to produce an opposite and substantially equal effect on said output, whereby the effect of said pulsation on said output is substantially eliminated.

2. In combination, an amplifier comprising a plurality of cascade connected electron discharge devices, each having a control electrode, means to transmit a gain control potential having an undesired ripple thereon to at least one of said electrodes, whereby the output of said amplifier is affected undesirably by said ripple, and means to transmit an amount of said undesired ripple to another of said electrodes sufficient substantially to cancel the effect of said ripple on the output of said amplifier.

3. In combination, a pair of cascade connected electron discharge devices each having a control electrode, means to transmit to said control electrodes in parallel a control potential having an undesired ripple thereon, whereby changes in said control potential change the amplification of said devices in the same sense and said ripple affects the output of, said amplifier, and means to adjust the amount of said ripple transmitted through one of said control electrodes to said output to produce an effect therein substantially equal and opposite to the effect produced by said ripple transmitted through the other of said control electrodes.

4. An amplifier having an output and a pair of cascade connected electron discharge devices, each having control means therefor, means connected to said output to derive therefrom a gain control potential having an undesired ripple thereon, means to apply said potential to at least one of said control means, whereby the gain through said amplifier is adjusted in accordance with said output, and means to transmit said undesired ripple to the other of said control means in an amount sufficient substantially to cancel the effect of said ripple on the output of said amplifier.

5. In a signal transmission system, a pair of cascade connected signal amplifying devices, means connected to the output of said devices for rectifying signals amplified therethrough to produce a pulsating unidirectional voltage, means to adjust the amplification of each of said devices in response to a unidirectional voltage, means for impressing said voltage on one of said adjusting means and for delaying the action of said adjusting means in response to changes in said unidirectional voltage an amount sufficient to prevent substantial distortion of said signals, said delaying means producing insufiicient delay to smooth said pulsating voltage, and means to impress the pulsations of said voltage on the other of said adjusting means in an amount sufficient to counteract the effect on said output of such pulsations impressed on said one adjusting means.

GEORGE M. BROWN. 

